Using a method we have developed for inserting one DNA molecule covalently into another, we will insert a variety of synthetic and natural, functionally defined DNA molecules into SV40 DNA. We will use these hybrid molecules (1) to insert new genetic information into mammalian cells in culture by transforming them with the hybrid DNA, (2) to determine whether bacterial genes can be expressed in mammalian cells, (3) to analyze this expression if it occurs, and (4) to construct a map correlating SV40 functions with position on the SV40 DNA molecule.